President Barack Obama has called for national unity after taking his public oath of office on the steps of the US Capitol to mark the beginning of his second term.
Hundreds of thousands of spectators converged on Washington, while millions more tuned in around the world to witness the ceremonial swearing-in, and to listen live as Mr Obama delivered his inaugural address.
US Supreme Court Chief Justice John Roberts administered the oath as the 44th president placed his hand on two Bibles - one used by President Abraham Lincoln at his first inauguration and one used by revered civil rights leader Dr Martin Luther King Jr.
After taking the oath, Mr Obama used his speech to call for a divided nation to come together to right the nation's course, following a bitterly partisan election and lame-duck session of Congress.
"Now more than ever we should do this as one nation," he said, adding that Americans are made for this moment and can succeed "so long as we seize it together".
Bibles belonging to Abraham Lincoln and Dr Martin Luther King Jr were usedMr Obama touched on several issues in his roughly 18-minute speech, laying out an ambitious programme for his coming four years.
The president urged the country to join him in tackling a vast array of problems, from slowing climate change to honouring the dignity of men, women and children around the globe.
Mr Obama said the country must make hard choices to reduce its massive deficit. "But we reject the belief that America must choose between caring for the generation that built this country and investing in the generation that will build its future," he said.
With the ceremonial swearing-in complete, the president and First Lady Michelle Obama took part in a traditional parade before heading out to formal balls where the presidential couple danced for the cameras.
Spectators packed the National Mall in Washington to catch the eventAfter delivering his inaugural address and listening to patriotic musical selections and a poem written just for the occasion, Mr Obama attempted to savour his final inaugural before walking off the platform.
Turning around to look at the scene on the National Mall, filled with hundreds of thousands of people who braved chilly weather to be part of the ceremony, the president said: "I want to take a look one more time. I'm not going to see this again."
At the ball, Mr Obama paid tribute to America's military, before praising his wife.
"Some may dispute the quality of our president, but nobody disputes the quality of the First Lady," he said.
While Mr Obama was officially sworn in on Sunday, as required by law, the glitter of Inauguration Day still enlivened staid Washington.
The celebration was pushed to Monday because January 20 fell on a Sunday. That placed the grand ceremony on the US holiday marking the birthday of Dr King.
President Obama and First Lady Michelle enjoy the 'first dance' at the ballThe widow of slain civil rights activist Medgar Evers, Myrlie Evers-Williams, delivered the invocation prior to Mr Obama taking the oath.
Vice President Joe Biden also took the ceremonial oath to kick off his second term as Mr Obama's number two.
Former US presidents Bill Clinton and Jimmy Carter were among the dignitaries seated on the Capitol steps for the event.
Mr Clinton and Mr Carter are the only living Democrats who have occupied the White House. Mr Clinton was accompanied by his wife, Secretary of State Hillary Clinton, who is departing as Mr Obama's second term gets under way.
In addition to the key political figures on hand, several celebrities, including Jay-Z and Beyonce, were in attendance to give the inaugural a red carpet atmosphere.
As he enters his second term, Americans increasingly see Mr Obama as a strong leader, according to a survey by the Pew Research Center for the People & the Press.
The survey shows him with a 52% job approval rating - among the highest rankings since early in his presidency. His personal favourable rating, 59%, has rebounded from a low of 50% in the 2012 campaign against Republican Mitt Romney.